1. Field of the Invention
The present invention relates to a method for measuring the soot load in an exhaust gas system of a diesel engine using a sensor connected downstream of a particle filter having a sensor element as a measure of the functional capability of the particle filter, in which the soot load is measured resistively or capacitively using a sensor element, wherein electrodes are formed on the sensor element and a measurement voltage is applied between the electrodes.
2. Description of the Related Art
Soot sensors, which are used in combination with particle filters in the exhaust gas systems of diesel engines, are known. The sensor elements of such soot sensors have for comb-shaped electrode structures. As the soot cover of the comb-shaped electrode structure of the sensor element increases, the electrical resistance of the electrode structure decreases, with the result that in this way the soot covering of the sensor element, and therefore the state of the particle filter which is combined therewith, can be detected. For example, the difference between an intact particle filter and a damaged particle filter can be determined using such a soot sensor.
The soot covering of the comb-shaped electrode structure or the soot precipitation thereon is therefore determined through the change in the electrical resistance or change in the capacitance of the electrode comb structure, and is used to assess the functional capability of a corresponding particle filter.
Such sensors or soot sensors are provided, in particular, for the on-board diagnostics of diesel systems with particle filters and serve to detect defects, for example in the filter ceramics of the particle filter. Since these sensors do not represent any additional benefit for the diesel system, it is only possible to use sensors which do not significantly increase the system costs. The simplest type of sensor in this context is a ceramic element loaded with soot in the exhaust gas. As mentioned, in this context this loading is measured resistively or capacitively by electrodes. However, since the particle concentration downstream of the particle filter is very low, such sensors must have a very high sensitivity. The sensitivity of the type of sensor mentioned above is, however, not sufficient for such a very low particle concentration, especially since the sensor has to be heated in order to avoid condensation of water or of other non-relevant liquids. The increased temperature even reduces the precipitation rate of soot particles on the sensor surface.
The use of such sensors with resistive or capacitive measuring equipment therefore entails problems owing to the low level of sensitivity. For this reason, other types of sensor have been used that have sufficient sensitivity based on the principle of charging and concentration-dependent transportation of charge. In addition, optical sensors have been used. However, these concepts have the disadvantage that they are very complex and therefore very expensive. In addition, these concepts also involve fundamental technical problems which have not yet been solved.